1. Field of the Invention
The invention relates to an interference signal cancellation system which is used in a CDMA mobile communication system, and particularly to an interference signal cancellation system which can increase a communication capacity by suppressing interference.
2. Description of the Related Art
Conventionally, examples of an interference signal cancellation system are proposed in, for example, Japanese Patent Unexamined Publication (Kokai) Nos. Hei 7-131382 and Hei 7-273713.
In the examples, the accuracy is gradually increased by repeating an interference canceling operation several times. The error rate is improved by canceling mutual interference among plural users, so that the capacity is increased.
FIG. 7 is a block diagram showing the configuration of an interference cancellation system of the prior art. In FIG. 7, a received signal 1 is supplied to a delay device 2 and matched filters 9. An output of each matched filter 9 is multiplied in a coefficient multiplier 10 with the eigenvalue .lambda. of a cross-correlation matrix of input signals, and the multiplied output is sent to a first-stage correlation cancellation circuit 7. In the circuit, the output is supplied to a respreading device 11 and a delay device 13. The output of the respreading device 11 is subtracted in a subtractor 12 from an output of the delay device 2.
When plural codes are used, the subtraction is conducted for all the codes. The result of the subtraction is supplied via an inverse diffusion device 14 to a coefficient multiplier 15 to be multiplied with the eigenvalue .lambda. of the cross-correlation matrix of the input signals. The multiplication result and an output of the delay device 13 are added to each other in an adder 16 to be output as a first-stage demodulation result 17.
The output of the delay device 2 is output via a delay device 3 as a first-stage delayed received signal 4. An Mth-stage correlation cancellation circuit 8 is dependently connected to the rear of the first-stage correlation cancellation circuit 7. An Mth-stage input signal 5 is supplied together with an Mth-stage demodulation input 18 to the Mth-stage correlation cancellation circuit 8 which in turn outputs an Mth-stage delayed signal 6 and an Mth-stage demodulation result 19.
The operation of the prior art example will be described with reference to FIG. 7. The received signal 1 is a CDMA multiple signal. The corresponding matched filter 9 correlates diffusion codes of the users, and fetches the signal of the corresponding user while suppressing signals of the other users and thermal noises. The signal is multiplied in the coefficient multiplier 10 with the eigenvalue .lambda. of a cross-correlation matrix of input signals among the users. The multiplication result is rediffused in the respreading device 11, and supplied also to the delay device 13.
The signal of each user which has been rediffused is subtracted in the subtractor 12 from the signal which is obtained by delaying the received signal 1 in the delay device 2. The output of the subtractor is then subjected to inverse diffusion in the inverse diffusion device 14. The result of the inverse diffusion is multiplied in the coefficient multiplier 15 with the eigenvalue .lambda. of the cross-correlation matrix of the input signals. The multiplication result and the output of the delay device 13 are added to each other in the adder 16 to be output as the first-stage demodulation result 17. This result exists for each of the users.
The output of the delay device 2 is further delayed in the delay device 3 so as to be output as the first-stage delayed received signal 4. The block which receives the outputs of the delay device 2 and the multiplier 10, and which outputs the first-stage delayed received signal 4 and the first-stage demodulation result 17 constitutes the first-stage correlation cancellation circuit 7.
The Mth-stage correlation cancellation circuit 8 is configured in the same manner as the first-stage correlation cancellation circuit 7, and dependently connected to the rear of the first-stage correlation cancellation circuit 7. As the number of stages which are dependently connected is larger, performance is higher. Therefore, two or more circuits are usually connected in order to attain higher performance.
In this way, even an interference signal cancellation system of the prior art can suppress interference and allow the capacity to be increased.
In the prior art example described above, however, the interference canceling operation must be repeated in order to attain desired performance. Namely, processes such as inverse diffusion and diffusion which require large computational complexity are repeated, and hence computational complexity becomes enormous, thereby producing a problem that it is difficult to realize desired performance.